Ultrasound technology has been used for therapeutic and diagnostic medical procedures, which can include providing imaging of internal portions of a body. For example, devices have been proposed for ultrasound imaging within blood vessels to view the condition of the vessel and/or placement or condition of a device placed in the vessel, as well as to help to determine plaque volume and the degree of stenosis within an artery lumen. That information is often difficult to obtain through angiographic imaging and exterior ultrasound imaging, particularly in regions having multiple overlapping arterial segments.
In some examples of intraluminal ultrasound procedures, a catheter is fitted with a transducer. A wire guide is positioned within a body conduit through use of angiography or ultrasound and is used to safely direct the catheter through the anatomy. The catheter is slid over the wire guide and positioned near the farthest end of the wire guide. The transducer transmits and/or receives ultrasound waves.
Difficulties arise in constructing adequate catheters having transducers in very small sizes in order to safely reach particular body conduits, such as for example with intravascular applications. Problems with existing two dimensional designs include wire guide channels which block a portion of the acoustic window. Additionally, wire guide channels take up valuable space in the catheter that could otherwise be used by ultrasound hardware. Three options for transducers that can be fitted into a catheter each have their own difficulties when compared to an external ultrasound device.
The first option is a single element transducer. The size of a single element transducer is typically appropriate for body conduits (for example 1-2 mm diameter) and the sensitivity or signal-to-noise ratio is good. Single element transducers are limited because they require a mechanical rotating mechanism, have a fixed focus, and require a slip ring or rotary transformer which adds cost. Existing catheters with this design do not have over-the-wire or rapid exchange capability.
The second option is a linear or phased array which includes multiple elements with the elements aligned in an axial direction. Advantages of this option are that the transducer is very flexible and acoustic performance is usually good. It also does not require mechanical rotation. Disadvantages are that it does not provide 360° side-view imaging. The cost is usually higher than a single element transducer and it requires more coaxial cables which can add bulk and complexity. This presents particular problems for over-the-wire designs or rapid exchange designs as there is not much space available within the catheter.
The third option is a circular array transducer having multiple elements that are all side-facing. A circular array design does not require mechanical rotation, provides 360° cross sectional imaging, and the cost is typically similar to single element transducer designs. This design is particularly suited for over-the-wire or rapid exchange designs, however the imaging quality suffers compared to other designs.
Other problems exist in current catheter configurations. For example, many such devices provide at best an image of a cross section of tissue or other items of interest, i.e. a thin, disk-shaped slice of the interior of a body conduit with a portion in the center that is not within the range of the ultrasound beam. In some other devices, the ultrasound beam is directed at a fixed angle that is not substantially perpendicular to the longitudinal axis (e.g. at 45 degrees). In this case the imaged region is static and takes the form of a truncated portion of the surface of a cone. In either case, in order to visualize the entirety of a significant length within the body (e.g. surfaces or portions of tissue, or of devices), the device must be moved along that length, with respective images of cross sections taken at particular locations. Such movement may be inexact, and may include risks associated with blind insertion of the device through the vessel, as well as being slow. Typical pull back images take on the order of 30 seconds to perform (at a speed of about 0.1 mm/second). Additionally, any changes in the orientation of the transducer during pullback distort the image.
Three-dimensional information provides added value useful for navigation and confirmation of position of devices within body conduits. In an intravascular example, catheters can be moved within vessels and the image data obtained via ultrasound can be combined or otherwise processed in order to create 3D information. A limitation of this technique is that it does not provide real-time information, so it cannot help with device delivery, but rather assists only with assessment of the device placement after delivery. Additionally, the catheter tip motion and angle must be known in order to produce accurate and usable data.
Three-dimensional images may be acquired by one-dimensional arrays connected to a mechanical actuator which moves the arrays within the catheter or other device. Such designs are expensive and generally require more space in a device than many vessels will permit. To achieve good image quality, such array transducers must simultaneously transmit and receive on many separate channels. That condition requires many expensive and bulky coaxial cables. Fewer coaxial cables can be used, but doing so reduces the quality of the image and image frame rate.
Ultrasound devices have been proposed which include a motion of a transducer about two axes to provide 3D information. However, in many devices the mechanical mechanisms that provide such movement tend to be bulky and require dimensions which are unsuitable for applications in catheters or small body areas. These problems are magnified when attempting to place a wire guide channel within the catheter. Proposed 3D or forward-looking transducer systems that are over the wire include a ring-array of very small transducer elements around the catheter lumen. However, such designs involve complex connections in small spaces which are accompanied by problems with wiring, cost and manufacturing. As a result, the connections are typically minimized and the image quality suffers accordingly.
There remains a need for a catheter placeable over a wire guide which can provide accurate and efficient application of ultrasound in three dimensions along a substantial length of a small body conduit. There also remains a need for such a device that can view a medical device and one or more tissues or tissue parts simultaneously, particularly in cases in which the device and tissue(s) could not have been imaged reliably in any two-dimensional plane.